Polyoma viruses with nucleotide alterations at specific sites in the early region are being constructed. Numerous mutants with deletions near the origin of DNA replication have been isolated and partially characterized. Some of these are altered in their ability to replicate DNA, or to express gene products. These mutants are being sequenced. Other mutants with alterations in the coding sequences for middle and large T-antigen, or for large T-antigen alone, have been isolated and partially characterized. The biological properties of these viruses are under study.